1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image on the recording medium using the nip portion formed by a fixing member and pressure member.
2. Description of Related Art
In an image forming apparatus using an electrophotographic process such as a photocopier, printer, facsimile and multi-functional peripheral having the functions of these devices, the latent image corresponding to a document is formed on a photoreceptor, and toner is applied to this latent image, whereby the image is developed. The developed toner image is transferred onto a recording sheet. After that, the toner image transferred onto the recording sheet is fixed and the sheet is ejected.
One of the fixing devices for fixing a toner image in the aforementioned manner includes a fixing device based on the heat roller fixing method, wherein the recording sheet with the toner image transferred thereto is sandwiched and transferred, using a nip portion formed between a fixing roller with a halogen heater built therein, and a pressure roller for applying pressure to the fixing roller, and, at the same time, this recording sheet is exposed to heat and pressure. Such a fixing device has been employed over an extensive range because of the simple structure.
Another example of the aforementioned fixing devices is a fixing device of the belt fixing method, wherein an endless fixing belt is applied to a heating roller with a halogen heater or the like built therein and a fixing roller, the aforementioned fixing device being provided with a pressure roller for applying pressure to the fixing roller through the fixing belt, and the recording sheet with toner image transferred thereto is sandwiched and transferred, using a nip portion formed between the fixing roller and pressure roller, while the aforementioned recording sheet is exposed to heat and pressure at the same time. Such a fixing device characterized by a smaller thermal capacity of the fixing belt reduces warm-up time and saves power.
In this case, the toner of the toner image on the recording sheet is heated in the process of passing through the nip portion. Thus, the toner acts as an adhesive; therefore, the recording sheet having passed through the nip portion sticks to the surface of the fixing roller and fixing belt and winds around the same without getting separated. This may cause a paper jam to occur Separability is further reduced when a sheet of a smaller basis weight (thin paper), particularly when the coated paper of reduced basis weight for printing is used as a recording sheet.
In the meantime, if the fixing roller is increased to ensure a nip width of sufficient size to cope with the increasing speed of the image forming apparatus, the roller curvature at the fixing nip outlet is reduced, with the result that sheet separability is further reduced.
To facilitate separation of the recording sheet from the fixing member, various measures have been taken. For example, the surface layer of the fixing member is made of a heat-resistant resin with high release characteristics, or is coated with a mold releasing agent such as silicone oil. Alternatively, the toner is impregnated with the wax that acts as a mold releasing agent when made molten by heat. However, there have been a growing number of factors for reducing the separability, including formation of an image on the coated paper, and an increase in the toner adhesive power caused by an increasing amount of toner due to the need of overlapping the toners of a plurality of colors for forming a color image. This makes it essential to use a separation assisting device.
In one of the means for assisting separation, a separation claw coated with fluorine resin characterized by excellent release characteristics is provided on the side of rejecting the recording sheet for the nip portion, and the leading edge thereof is brought in contact with the outer surface of the fixing roller or fixing belt, whereby the recording sheet is separated from the fixing roller and others.
However, the leading edge of the separation claw is in contact with the surface of the fixing roller or others. Accordingly, the surface layer formed of the fluorine resin and others coating the fixing roller and others is scratched, and the scratch is transferred to the image. This problem tends to be serious since a glossy image is required especially in the case of a color image.
One of solutions to this problem is a technique wherein air is blown on the outside side of the nip portion to separate the recording sheet from the fixing roller and others.
One example of this technique is found in a sheet separation device wherein compressed air provided by a compressor is stored in two air boxes, and two electromagnetic valves connected to the air boxes are alternately turned on and off so as to jet compressed air and to meet the requirements of a high-speed photocopier (Unexamined Japanese Patent Application Publication No. Sho 60 (1985)-256180).
Another example is a fixing device wherein a separation claw (separation claw) is provided and the air fed by a fan is blown on the nip portion (Japanese Utility Model Laid-Open No. Sho 63 (1988)-140571).
Further, another known example is a separation device wherein a separation assisting plate is arranged close to the nip portion and spiral-formed compressed air is jetted out from between the fixing roller and separation assisting plate (Unexamined Japanese Patent Application Publication No. 2004-212954).
Another known example is a fixing device wherein the compressed air produced from a compressor is jetted out by two electromagnetic valves and high-pressure compressed air is jetted when the leading edge of the recording sheet has passed through the nip portion. After that, the compressed air of lower pressure is blown thereafter (Unexamined Japanese Patent Application Publication No. 2007-86132).
When air is blown so that the fixed recording sheet will not stick to the fixing roller or others, and the recording sheet is separated from the fixing roller, the effect is influenced by the area of the portion on which air is blown. If no toner image is formed close to the leading edge of the recording sheet and there is a wide area free from adhesive force, the leading edge of the recording sheet is separated by the toughness and weight of the recording sheet, and air is received by the separated area. This provides a greater separation force. However, when a toner image has been formed up to the leading edge of the recording sheet, the recording sheet is conveyed in the direction tangential to the outer periphery of the fixing roller and others. This minimizes the clearance between the leading edge of the recording sheet and the outer periphery of the fixing roller and others. For example, when the outer diameter of the fixing roller is 90 mm and the margin of the leading edge of the recording sheet is 3 mm, the clearance is only 0.1 mm. To blow air into this clearance and to lift the leading edge of the recording sheet, it is necessary to blow air at a high velocity, namely, high pressure air to the nip portion. To achieve this, it is preferred to use the high-pressure compressed air produced from a compressor.
The recording sheet having passed through the nip portion sticks to the surface of the fixing roller or fixing belt and winds around the same without being separated. This phenomenon occurs not only at the leading edge of the recording sheet but also after the leading edge. Further, even if the leading edge of the recording sheet is pulled out by the sheet ejection roller, irregular separation occurs at the wound portion. Accordingly, when air is blown on the outlet side of the nip portion to separate the recording sheet from the fixing roller and others, air must be blown on a continuous basis.
If compressed air is blown on a continuous basis in this manner, compressed air close to 0.01 m3/s will be required. A great amount of energy is required to produce compressed air. To produce the compressed air of this air volume, a large-power compressor with a capacity of 5 to 10 kW must be used. The size of the device including a compressor and air tank may reach a high level of about 1 m3.
In the conventional literatures, it is difficult to find a structure that meets the related performances without allowing the size of the device to increase.
To be more specific, the Unexamined Japanese Patent Application Publication No. Sho 60 (1985)-256180 may result in producing a large-sized device including a large-power compressor, as described above.
The Japanese Utility Model Laid-Open No. Sho 63 (1988)-140571 uses the air fed by a fan. In this case, the high pressure air cannot be obtained, and use of a combination with a separation claw is inevitable. Accordingly, the surface layer of the fixing roller will be scratched by the separation claw. This problem has not been solved.
In the Unexamined Japanese Patent Application Publication No. 2004-212954, after the leading edge of the recording sheet has been removed by compressed air, the recording sheet is separated by a sharp separation assisting plate. This may cause the image of the recording sheet to be scratched.
In the Unexamined Japanese Patent Application Publication No. 2007-86132, after the leading edge of the recording sheet has been removed by compressed air, the pressure of the compressed air is reduced. However, to maintain separability, a sufficient amount of air is required. To ensure this amount of air, a large-sized device equipped with a large-power compressor will have to be produced, as described above.
In view of the problems described above, it is an object of the present invention to provide a fixing device and an image forming apparatus provided with this fixing device, wherein this fixing device separates the leading edge of a recording sheet by compressed air, without having to use a large-power compressor or allowing the device to be increased in size.